It is increasingly common with machine tools to include electronic devices which measure and digitally display the machine's coordinates, independent of the lead screw dials. Such devices increase productivity, since they are easy to read and are not subject to inaccuracies from machine wear.
They typically consist of several axes of optical or magnetic distance scales coupled to a common display box. The display in turn typically contains a keypad for entering preset distances, along with several channels of digital readout for displaying the current coordinates of the cutting tool, and a processor. The processor accumulates pulses from the scales, and provides such functions as English-to-metric units conversion. Representative of these devices is the apparatus described in U.S. Pat. No. 3,872,288.
Even with these devices, however, some arithmetic drudgery is necessary on the part of the machinist. This comes about because his display shows the coordinates of the center of his cutter, whereas the cutting is done by its periphery. The part drawings he works from specify the coordinates of the final cuts, which he then must convert to cutter centerline coordinates by adding in or subtracting out the radius of the cutter.
For example, to start a rectangular cutout in his workpiece at X=1.960 inches, with a 0.250 inch diameter end-mill, the machinist first sets his machine to a calculated X coordinate of 1.960+0.250/2=2.085 inches. Completing the cutout involves repeating similar computations three more times: a second addition to compute the Y coordinate, followed by a first and second subtraction to compute the X and Y coordinates of the opposite sides of the cutout.
It is not uncommon for one workpiece to require many dozen such calculations. Besides the time expended, there is always the risk of incurring a mathematical error, which could likely ruin the part.
Some digital coordinate readouts partially solve this problem for the machinest by mechanizing the mathematics, if not the decision making function. Typically they contain a register into which the machinist makes a one-time entry of the cutter radius. One such product is the Bausch & Lomb ACCU-RITE.TM.. With it, control keys are provided which allow the cutter radius to be either added to or subtracted from the X or Y coordinates, at the manual direction of the machinest.
In practice this feature is seldom used, because it does not solve the total problem, and trades one set of problems for another. The operator must now divide his attention while machining, continually making offset-direction decisions, and reaching each time for the appropriate key.